1. Field of the Invention
The present invention relates to a thin film transistor allowing an element characteristic of the thin film transistor having a coplanar structure using an oxide semiconductor to be enhanced, a method of manufacturing the thin film transistor, and a display device including the thin film transistor.
2. Discussion of Related Art
As much attention has been focused recently on information displays and needs for a portable electronic device has been increasing, research and commercialization on a lightweight and thin film type display device and a large-sized high-resolution display device are widely made. In particular, among these various display devices, research on a liquid crystal display (LCD) and an organic light emitting display (OLED) is widely made.
In the LCD and the OLED, a thin film transistor (TFT) is used as a switching element and/or a driving element. The thin film transistor is classified as a thin film transistor using amorphous silicon, a thin film transistor using polysilicon, or a thin film transistor using an oxide semiconductor depending on a material used as an active layer. In the case of the thin film transistor using the polycrystalline silicon, a process of implanting ions is carried out to adjust a resistance of the active layer. An additional mask for defining an ion implantation region may be used, and the ion implantation process is added, thereby causing a disadvantage in terms of process. On the other hand, in the case of the thin film transistor using the oxide semiconductor, the electron mobility increases as compared to that of the thin film transistor using the amorphous silicon, an amount of the leakage current is significantly lower than that of the thin film transistor using the amorphous silicon and the thin film transistor using the polycrystalline silicon, and a high reliability test condition is satisfied. In addition, the thin film transistor using the oxide semiconductor can advantageously ensure that a distribution of threshold voltages is uniform as compared to the thin film transistor using the polycrystalline silicon.
The thin film transistor using the oxide semiconductor may be classified as a thin film transistor having an inverted-staggered structure or a thin film transistor having a coplanar structure depending on positions of an active layer, a gate electrode, a source electrode, and a drain electrode. Since the thin film transistor having the inverted-staggered structure has a high parasitic capacitance between the gate electrode and the source and drain electrodes, it is difficult to apply the thin film transistor having the inverted-staggered structure to a high-resolution display.
Inventors of the present invention have recognized that a high resistance occurred due to an interval of several micrometers between a portion in which the active layer and the source and drain electrodes are in contact with each other and a channel region of the active layer in the thin film transistor having the coplanar structure. To address this problem, the inventors have made the thin film transistor having the improved coplanar structure.